The bonding of integrated circuit leads is a process requiring both control over physical placement of the integrated circuit leads as well as the laser energy directed at those leads to bond them. When a laser or other source of directed energy is used to supply the energy required to bond the leads to bonding pads or other sites, control over placement of the beam is critical to avoid damaging the integrated circuits substrates located around the bonding pads or other sites. Damage to the substrate of a circuit board or other electrical component can lead to failure of the circuit board or component.
The importance of accuracy or masking of the laser beam becomes apparent when the relative sizes of the leads, bonding surfaces and tools normally used in the bonding process are considered. The state of the art integrated circuit lead has a width of less than 0.004 inches while the tools used to hold down and bond leads to bonding surfaces are typically less than 0.008 inches wide.
Focusing of laser energy to reduce spot size also involves difficulties. Every attempt to focus a laser beam results in a spot with accompanying fringe energy outside the spot diameter. That fringe energy is intense enough outside the spot diameter to damage the substrate and, when combined with other focusing problems such as aiming of optics as well as X-Y translation table inaccuracies, substrate damage is very difficult to eliminate.
Prior attempts to use lasers and other forms of directed energy to bond leads and bonding surfaces have included the use of hollowed tools with the energy being directed down the center of the hollowed tool. Problems with that arrangement included tools that were too large to be useful in bonding smaller sized leads to bonding surfaces, fabrication difficulties due to the small hole diameter versus length of hole required, splattering of metal and other materials caused by heating of the leads and bonding surfaces within the hollowed tool, and thermal cascading of the bond tool. Splattering, in particular, reduces the effectiveness of the energy beam by coating the interior of the hollowed tool, narrowing the through-hole and blocking energy from reaching the lead. The small through-holes also absorb a large portion of the energy before splatter reduces their diameters. Thermal cascading of the bond tool results in bond strength variations, which are unacceptable for high volume processing.
The present invention provides a method and apparatus for masking a laser or other directed energy beam used in bonding leads while avoiding the problems faced with prior art devices. The present invention uses a modified standard tool which allows standard lead bonding equipment to be used in conjunction with a laser or other energy source and the modified tool. That results in additional reliability and dependability because of the standard equipment used to bond the leads.